fix(ci): use the intended Python interpreter for CMake discovery (#6105)

* fix(ci): pin the discovered Python to the venv/setup-python interpreter

The manylinux/musllinux images added Python 3.15 and dropped 3.13t;
FindPython's version scan then picked /usr/local/bin/python3.15 over
the uv-created venv (Python_ROOT_DIR is only a hint). Similarly, the
macOS and Windows 3.14t jobs picked a system 3.14 over setup-python's
free-threaded interpreter. Pin the interpreter explicitly in both
places, and drop the 3.13t manylinux jobs.

Assisted-by: ClaudeCode:claude-fable-5

* ci: use setup-python's python-path output instead of a locate step

Assisted-by: ClaudeCode:claude-fable-5

* Apply suggestions from code review

Co-authored-by: Ralf W. Grosse-Kunstleve <rwgkio@gmail.com>

---------

Co-authored-by: Ralf W. Grosse-Kunstleve <rwgkio@gmail.com>

.github/workflows/ci.yml

@@ -240,10 +240,6 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - container: quay.io/pypa/manylinux_2_28_x86_64:latest
-            python-version: '3.13t'
-          - container: quay.io/pypa/musllinux_1_2_x86_64:latest
-            python-version: '3.13t'
           - container: quay.io/pypa/manylinux_2_28_x86_64:latest
             python-version: '3.14t'
           - container: quay.io/pypa/musllinux_1_2_x86_64:latest

.github/workflows/reusable-standard.yml

@@ -34,6 +34,7 @@ jobs:
       - uses: actions/checkout@v7
 
       - name: Setup Python ${{ inputs.python-version }}
+        id: python
         uses: actions/setup-python@v6
         with:
           python-version: ${{ inputs.python-version }}
@@ -67,6 +68,9 @@ jobs:
         if: runner.os != 'Windows'
         run: echo "CMAKE_GENERATOR=Ninja" >> "$GITHUB_ENV"
 
+      # Pass setup-python's resolved interpreter path to CMake so FindPython
+      # does not select a different interpreter (e.g. Homebrew's 3.14 instead
+      # of setup-python's 3.14t).
       - name: Configure
         run: >
           cmake -S. -Bbuild -Werror=dev
@@ -74,6 +78,7 @@ jobs:
           -DPYBIND11_PYTEST_ARGS=-v
           -DDOWNLOAD_CATCH=ON
           -DDOWNLOAD_EIGEN=ON
+          -DPYTHON_EXECUTABLE=${{ steps.python.outputs.python-path }}
           ${{ inputs.cmake-args }}
 
       - name: Build